1. Field of the Invention
The present invention relates generally to improvements of a power source which utilizes a piezoelectric transformer, and more specifically to a hardware arrangement and method of effectively driving a piezoelectric transformer which provides a load with a high ac (alternate current) voltage. The present invention is well suited for use with a cold cathode fluorescent lamp provided in a backlit display of a notebook type computer, however, is in no way limited to such usage.
2. Description of the Related Art
In order to reduce a size of a power source used for providing a high ac voltage, it has been proposed to employ a piezoelectric transformer in place of a conventional electromagnetic-type transformer. When a piezoelectric transformer is used in a power source for generating a high ac voltage, it is known to use a feedback control loop to attain stable operations of the device. By way of example, such prior techniques are disclosed in Japanese Laid-open Patent Applications Nos. 61-152165, 4-49846, 4-133657, and 5-219730.
These known techniques disclose various feedback techniques for controlling a driving frequency in response to output voltage and/or current of the transformer. However, it is necessary to ensure stable energization of the piezoelectric transformer when it is initially put into operation. Further, it is very important to prepare for undesirable occurrences which can be encountered while the transformer is operating normally. For example, it is necessary to restore stable operation of the transformer when a load current decreases due to an unexpected reduction of the main dc voltage of a system. Still further, it is very important to prevent the piezoelectric transformer from damaged when the transformer is accidentally disconnected from a load while operating in a stable mode.